Some of the fish in Mildred Lake in Yosemite National Park have high enough concentrations of mercury in their tissues to be a danger to your health.

A study announced Wednesday by the U.S. Geological Survey and the National Parks Service found that fish in a number of national parks in the western United States have mercury levels high enough to be a danger to wildlife or humans.

But the single highest mercury concentration found and the only case in which that concentration was so high that health guidelines advise against anyone consuming the fish at all were for a single brook trout at Mildred Lake.

The study tested 1,400 fish from 86 lakes and streams in national parks across the western United States between 2008 and 2012. Many lakes in national parks are in high-altitude locations that generally have less risk of being contaminated with mercury than do lower-elevation waterways.

And the study found that concentrations of mercury in the tissues of fish from the national parks on average are lower than for fish elsewhere in the western states.

"Although fish mercury concentrations were elevated in some sites, the majority of fish across the region had concentrations that were below most benchmarks associated with impaired health of fish, wildlife, and humans," said USGS ecologist Collin Eagles-Smith, the lead author of the publication.

Still, that finding won't protect people who happen to eat a fish that does have high mercury concentrations.

What's surprising, said an expert on pollution and sportfishing in California, is that a high altitude lake in Yosemite National Park would have a fish with such high mercury levels that it shouldn't be eaten at all.

"Unless there had been some historic mining up there or something, I am puzzled," said Bill Jennings, the executive director of the California Sportfishing Protection Alliance. "I would be curious as to what sort of historical activities occurred pre-park protection in the old Gold Rush era."

In many parts of California, as well as elsewhere, waste left from gold mining releases mercury into the environment. Mercury was used to process gold ore.

Different state and federal agencies have come to different conclusions about how much mercury is safe in fish consumed by humans. The USGS study of national park fish primarily considered standards developed in the Great Lakes region.

Mercury contaminates many fish in the Great Lakes area. As a result, state governments have cooperated with each other and the federal Environmental Protection Agency for decades to advise the public on how much fish is safe for people to eat.

In general, the Great Lakes Advisory Group recommends unlimited consumption only of fish with less than 50 parts per billion (also sometimes expressed as less than 50 nanograms/gram wet weight) of mercury.

The EPA, in contrast, recommends that women of childbearing age and children stick to fish with less than 300 parts per billion of mercury. The EPA guideline is based on eating two or three servings of fish a month.

Mercury can damage the neurological development of children, which is why pregnant women and children are particularly urged to avoid consuming types of fish that are high in mercury.

The Great Lakes guidelines recommend that fish with more than 950 ppm of mercury should never be eaten by humans.

One brook trout caught in Mildred Lake in 2009 clocked in at 1,109 ppm. The fish tested in Mildred Lake averaged 174.4 ppm. In contrast, fish tested at Spillway Lake, also in Yosemite National Park, averaged just 38.7 ppm, or well below the level considered safe for humans.

A staffer reached by phone Wednesday in Yosemite's public information office said he had not yet heard of the USGS study. He referred questions to Park Information Officer Scott Gediman. Gediman did not respond Wednesday to email and telephone messages asking for comment.

Contact reporter Dana M. Nichols at (209) 607-1361 or dnichols@recordnet.com. Follow him at recordnet.com/calaverasblog and on Twitter @DanaReports.